We have employed reflection-absorption infrared spectroscopy (RAIRS) as the principal technique as well as low energy electron diffraction (LEED) and thermal programmed desorption (TPD) as diagnostic tools to study the adsorption of CO, NO, NO/O, NO/O+N, and NO/K on Co{1010}. A novel reversible order-order phase transition is revealed by studies of a chemisorbed monolayer of CO on Co{1010} between 100 and 150 K. The high temperature phase consists of tilted 2-fold bridging CO molecules in the well established p(2x1)g structure, while the low temperature phase involves the movement of one-third of the CO molecules into 3-fold hollow sites to produce a new p(6x1)g structure. The temperature-induced site switching is attributed to a vibrational-entropy-driven secondary-order displacive phase transition. The high coverage structures of CO on Co{1010} have also been investigated over the temperature range for 100 to 250 K. This has revealed a curious anomaly. As the coverage is increased above 0.5 monolayer (ML) at temperatures below 250 K, a p(2x1) phase, with atop CO, is incompletely converted to a p(2x1) phase with a local coverage of 1 ML, and CO in bridge sites. At temperatures below 180 K, one third of the surface is converted into the energetically most stable structure, c(2x6), with Co in two types of bridge sites, local coverage1.17 ML, and the remaining two thirds remains in the p(2x1)g phase. On cooling to 100 K, the stable c(2x6) phase is unchanged, still occupying one third of the surface, and the p(2x1)g phase is transformed to a p(6x1)g phase, driven by vibrational entropy. At these low temperatures, this phase transition occurs between two ordered phases which are both metastable with respect to the high coverage (c(2x6) phase. This is attributed to kinetic constraints within the close-packed adsorbed overlayer associated with frozen-in antiphase domains. For the interaction of NO with clean Co{1010} at 100 K, NO is found to absorb molecularly on the Co{1010} surface with an estimated 0.5 ML coverage at saturation. At low coverages, three adsorption sites, on-top, two-fold and three-fold, are occupied; while from moderate coverage to saturation, bridge-bonded NO becomes dominant. A sharp c(2x4) LEED pattern is observed at saturation, and a corresponding surface structure is proposed for the pattern. For all precoverages of NO dissociation is complete on heating the surface, but the temperature at which dissociation occurs depends critically on NO coverage and on the presence of the dissociation products. TPD spectra indicate that no species other than N<SUB>2</SUB> desorbs below 630K; c(2x6) and p(2x3) LEED patterns are observed after annealing to 600 K, and removal of O adatoms by H<SUB>2</SUB> show that these are due to N adsorption. A first-order unimolecular reaction mechanism is proposed for NO dissociation on Co{1010}, with a strong dependence of the activation energy for surface dissociation on both NO coverage and the coverage of the dissociation products, N and O. For the adsorption of NO/O on Co{1010} at 100 K, the presence of oxygen adatoms greatly attenuates the occupation of two-fold sites in favour of atop sites, but O adatoms do not show any significant blocking effect for NO adsorption, and the NO coverage is close to 0.5 ML, as found on the clean surface. The coadsorption of NO with various precoverages of oxygen (0.15 - 1.0 ML) including three ordered oxygen overlayers, c(2x4) (θ ≈ 0.5 ML); p(2x1) (θ ≈ 0.5 ML) and p(2x1)-g (θ ≈ 1.0 ML) , reveals a NO-induced surface restructuring process, in which O adatoms are driven from overlayer to underlayer sites at high NO coverages, and this restructuring process increases the O effective diameter to up to ~ 10 Å.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599770 |
| Date | January 2001 |
| Creators | Gu, J. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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